The present invention relates to compositions embodying phase change materials and, more particularly, to compositions containing crystalline, long chain alkyl hydrocarbons having at least 14 carbon atoms.
There is a great deal of interest in phase change thermal energy storage systems due to their inherent ability to store large amounts of heat and release it to the surrounding environment as temperatures drop below or exceed predetermined levels. These systems are of particular interest in the architectural and building trades where climate control and its concomitant energy consumption is one of the principal considerations in building design and material selection.
A variety of building materials and techniques have previously been used to conserve heat or cool and thereby reduce energy costs. Included among them are structural elements which incorporate phase change materials. By incorporating phase change materials into building materials, energy in excess of that necessary to maintain comfort conditions is inherently absorbed and released as required to maintain the comfort range. Thus, in winter months, phase change materials incorporated into structural elements in the walls or floors of buildings and the like can absorb solar energy during daytime hours and release it to the interior at night as temperatures drop. In summer months, the same phase change material, due to its thermostatic character, conserves coolness by absorbing energy.
Structural elements incorporating phase change materials are more desirable than elements which store only sensible heat because they have a higher capacity to store energy and they absorb and release a large quantum of energy over a very narrow temperature range. A phase change material utilizes its latent heat of fusion as well as its sensible heat capacity for thermal storage. The latent heat of fusion is substantially greater than the sensible heat capacity of the material. That is, the amount of energy a material absorbs upon melting, or releases upon freezing, is much greater than the amount of energy it absorbs or releases upon increasing or decreasing in temperature 1.degree. C. Thus, upon melting and freezing, per unit weight, a phase change material absorbs and releases substantially more energy than a sensible heat storage material which is heated or cooled through the same temperature range. Furthermore, as contrasted with a sensible heat storage material which absorbs and releases energy essentially uniformly over a broad temperature range, a phase change material absorbs and releases a large quantum of energy in the vicinity of its melting/freezing point. This is particularly advantageous in buildings where space is at a premium and energy storage and release are required within a very narrow comfort range.
It has long been recognized that an effective phase change material, which could store and release thermal energy within the temperature range of 10.degree.-65.degree. C. and could be economically incorporated into common building materials (e.g., concrete, cement, plaster, rubber, plastics), would have broad utility for many heating and cooling applications including solar passive, solar active, off-peak electric load leveling, bridge deck deicing, etc. Other types of phase change materials have been investigated that melt and freeze in the above temperature range, and have a high heat of fusion (e.g., salt hydrates and clathrates); but widespread use has not been achieved because of the difficulty of containerizing them, their instability to repeated thermocycling, corrosion, leakage, etc.
Paraffin waxes have been considered for use in building materials as phase change materials but until now effective methods of incorporating them into building materials were not available and/or they involved prohibitive loss in the physical properties of the building materials.
Among the teachings which were available in the art prior to the present invention are those of U.S. Pat. No. 4,259,401 to Chahroudi et al which discloses both structural and non-structural building materials incorporating phase change materials. These building materials are made up of a rigid porous matrix structure which is impregnated with the phase change material. Three classes of phase change materials are disclosed, namely, hydrated salts, waxes, and clathrates. Cements, plasters or thermosetting materials may form the rigid matrix.
U.S. Pat. No. 4,504,402 to Chen teaches an encapsulated phase change material which is prepared by forming a shell about a phase change composition in compacted powder form. One of the applications of the encapsulated phase change materials is in concrete or gypsum structures.